Concrete workability

One of the basic attributes of any concrete is its workability or “consistence”, that is, how easy it is to push one way, pull the other, and float to a smooth level surface. Workability is largely determined by how fluid the concrete is. This is referred to as “Slump”. Essentially, the more fluid the concrete, the higher the slump. Although slump is often seen as an indication of water content, it is more legitimately interpreted as a measure of consistence.

Following the adoption of the new European Standard for Concrete in 2003 (BS8500), consistence (workability or slump) is now specified as being of a particular class.

Concrete slump values

There are five classes with each class spanning a range of slump values.

Class Slump range (mm) Target slump
S1 10-40 20
S2 50-90 70
S3 100-150 130
Table 1: Slump class from BS8500

A rough guide to useage

S1 slumps are most likely to be used for kerb and pipework bedding because it’s quite a dry concrete mix. This is sometimes referred to as “semi-dry”. 

S2 slumps are used for simple strip footings and cast in-situ hard-standing slabs or when using a wheel barrow to move the concrete. This is referred to as a “moist mix” and probably the most useful and most commonly specified consistence.

S3 would be used for trench-fill foundations where a high flow is required. This is known as a “wet mix”.

What is a slump test?

The slump of a concrete (or mortar) is determined via a fairly simple test using incredibly simple equipment. This testing apparatus is readily available from contractor’s tool suppliers, although all ready mix concrete suppliers will test the slump of their delivered product on request.

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